The invention relates to a medullary-space nail for the surgical treatment of fractures of thin tubular bones in the human body.
Plates and screws are ordinarily used in the surgical treatment of forearm fractures. Contrary to the case of medullary nail treatment in other tubular bones (tibia, femur and humerus), treating the ulna or radius with medullary nails has not been widely accepted to date. In some cases pairs of thin medullary nails have been used and some bone splinting is made possible. However, rotation of the bone fragments cannot be prevented because the nails are not connected to each other. Moreover the medullary nails are affixed only by tensioning in the medullary space and outward migration of thin nails may raise problems. Experiments carried out with conventional locking nails have failed because a relatively small nail cross-section permits only minute cross-boreholes which are difficult to locate when the nail is inserted into the human body, even when using an image intensifier. Also, locking pins used with such nails are generally too small to absorb the generated forces.
One known medullary space nail is disclosed in U.S. Pat. No. 4,805,607 and has a non-circular, preferably triangular cross-section with concave sides. Using a reamer, this configuration of the medullary nail allows implantation in the medullary space of a bone without pre-drilling the medullary canal. The triangular nail allows cortical contact along three very thin metal edges. This localized configuration of the cortical contact reduces the danger that the medullary nail will rotate in the medullary space.
Another known medullary nail is disclosed in U.S. Pat. No. 5,116,335 and has a core and three flexible rods displaceable relative to the core. The core comprises three grooves situated on the circumference of the core spaced at 120xc2x0 relative to each other and extending over the entire length of the core. Along the length of the core, the grooves have a cross-section that reduces toward the outer surface of the core and holds the rods. At the proximal and distal ends of the core, the cross-section of the grooves does not taper toward the outer surface, and the rods may be bent away transversely to the central axis for anchoring within the bone.
Yet, another medullary nail for the surgical treatment of forearm fractures, comprises a starshaped nail cross-section, and is marketed by Applied Osteo Systems Inc. of Walnut Creek, Calif. under the name of Trueflex Nail. Aside from splinting the bone, the star-shaped nail cross-section is intended to prevent the main fragments from rotating. However this result is only possible if the nail is struck as far as possible into the medullary space to anchor the nail tip in the spongy substance near the joint. However, the fracture zone can be stretched and fracture healing may be interfered with.
The medullary nail of the invention comprises a nail shank with channels running parallel and eccentrically to the longitudinal axis. The channels receive locking wires running parallel to the medullary nail and are rotatable inside the channels and can be translated in the longitudinal direction of the medullary nail. The channels define an opening along a longitudinal portion of the outside surface of the nail shank and the locking wires are prevented from moving out of the channels transversely to the longitudinal axis.
In one embodiment, the channels have an inside thread in the region of the lower end. In a preferred embodiment, the nail shank defines a throughbore in the vicinity of the upper end, and the throughbore has a central axis extending transversely to the longitudinal axis for receiving a locking screw therethrough to preclude displacement of the nail shank relative to the longitudinal axis as well as rotation of the nail shank about the longitudinal axis. The channels can have a recess in the region of the thoughbore, and the locking wires can be bendable around the locking screw.
In one preferred embodiment, the locking wires comprise threaded tips corresponding to the inside threads in the channels, and the tips are displaceable parallel to the longitudinal axis beyond the nail tip and screwable into the bone.
In another embodiment, the stem has an oblate cross-section with a perimeter including two parallel chord segments offset by a first distance and a pair of opposite semicircular surfaces having a first diameter. In one embodiment, the ratio of the first distance to the first diameter is in a range of about 0.50 to about 0.80. In another embodiment, the ratio of the first distance to the first diameter is in a range of about 0.60 to about 0.70.
In yet another embodiment, the channels are fitted into the peripheral arcs of circle of the cross-section. The end part can have a circular cross-section. The nail shank can be made of titanium, stainless steel, or plastic